Method and apparatus for determining relative head-to-disk speed

ABSTRACT

A system including a write head, a read head, a mixer, and a filter. The write head is configured to write a pattern on a track of a medium of a hard disk drive, wherein the pattern has a first frequency. The read head is configured to read the pattern written on the track of the medium of the hard disk drive. The mixer is configured to mix a first signal generated by reading the pattern with a second signal to obtain a mixed signal. The second signal has a second frequency, and wherein the second frequency is different than the first frequency. The filter is configured to filter the mixed signal to determine a relative head-to-disk speed. The filter has a parameter selected based on a difference between the first frequency and the second frequency.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present disclosure is a continuation of U.S. patent application Ser.No. 13/891,735 filed on May 10, 2013, which claims the benefit of U.S.Provisional Application No. 61/645,140 filed on May 10, 2012. The entiredisclosures of the applications referenced above are incorporated hereinby reference.

FIELD

The present invention generally relates to data storage devices, andmore particularly relates to hard disk drive (HDD) data storageproducts, including methods and apparatus for monitoring and adjustingthe operation thereof.

BACKGROUND

Hard disk drives include a recordable medium and a head which is mountedto move above the recordable medium as the medium spins. As hard diskdrives are designed to smaller and smaller sizes and the medium isdesigned to store more and more information on narrower and narrowertracks, it is imperative that the relative positions of the rotatingmedium and the moving head be monitored and other parameters such asrelative motion and speed variations be monitored. Such monitoring isneeded in order to correct any errors before they become too great.However, today's monitoring techniques are insufficient for the trackwidths, height tolerances and other parameters.

Thus, what is needed are robust monitoring and self-adjustmenttechniques for hard disk drives that are compatible with futureultra-thin, greater storage hard disk drives. Furthermore, otherdesirable features and characteristics will become apparent from thesubsequent detailed description, taken in conjunction with theaccompanying drawings and this background of the disclosure.

SUMMARY

According to the Detailed Description, a method for monitoring hard diskdrive operation in a hard disk drive system is provided. The hard diskdrive system includes a spindle, a disk and a head. The method includesthe steps of writing a wide pattern having a predetermined frequency ona track of a hard disk drive medium, generating a readback signal byreading the wide pattern from the track, processing the readback signalby mixing the readback signal with a reference signal to obtain a summedsignal and a difference signal, and filtering the summed signal by afilter centered around the difference signal to generate a measurementsignal corresponding to a relative speed change of the spindle and ahead-to-disk motion.

In accordance with another aspect of the present embodiment, a methodfor monitoring operation of a hard disk drive system is presented. Thehard disk drive system includes a disk medium including a buried servolayer having a first frequency of data on a first track and a secondfrequency of data on a second track, the second track adjacent to thefirst track. The method includes the steps of reading a readback signalby a head positioned at the middle of the first track and the secondtrack, filtering the signal by a first filter centered at the firstfrequency to extract a first component signal, filtering the signal by asecond filter centered at the second frequency to extract a secondcomponent signal, combining the first component signal and the secondcomponent signal to generate a synchronization signal comprisingdowntrack information. The downtrack information may include spindlespeed variation, spindle speed jitter, and relative head-disk vibrationin the downtrack direction.

In accordance with yet another aspect of the present invention, a methodfor measuring flying height of a head over a disk medium in a hard diskdrive (HDD) system is provided. The HDD system includes a disk mediumhaving a buried servo layer with a first frequency of data on a firsttrack and a second frequency of data on a second track, the second trackadjacent to the first track. The method includes the steps of reading areadback signal by a head positioned at the middle of the first trackand the second track, filtering the signal by a first filter centered atthe first frequency to extract a first component signal, filtering thesignal by a second filter centered at the second frequency to extract asecond component signal, combining the first component signal and thesecond component signal using Wallace equations to generate asynchronization signal comprising flying height information. A furtheraspect includes developing a table of operational parameters for variouspressures and temperatures from the flying height information forimproved HDD system operation.

BRIEF DESCRIPTION OF DRAWINGS

The accompanying figures, where like reference numerals refer toidentical or functionally similar elements throughout the separate viewsand which together with the detailed description below are incorporatedin and form part of the specification, serve to illustrate variousembodiments and to explain various principles and advantages inaccordance with a present invention.

FIG. 1 illustrates a system for real time spindle speed variation andrelative head-disk motion measurement in accordance with a presentembodiment.

FIG. 2 illustrates a double filter variant of the system of FIG. 1 inaccordance with the present embodiment.

FIG. 3 illustrates a graph of filtered signals of the variant system ofFIG. 2 in accordance with the present embodiment.

FIG. 4, comprising FIGS. 4A and 4B, illustrates experimental resultsfrom the system of FIG. 1 in accordance with the present invention,wherein FIG. 4A shows experimental results obtained using the system ofFIG. 1 on an air bearing spindle with a measured spindle speed variationand FIG. 4B shows experimental results obtained using the system of FIG.1 for measurements on a hard disk drive (HDD) product.

FIG. 5 illustrates experimental results obtained using the system ofFIG. 2 for measurements on the HDD product of FIG. 4B in accordance withthe present invention.

FIG. 6 illustrates a cutaway view of a read-write head over disk mediain accordance with the present invention.

FIG. 7 illustrates atop planar view of the reader head of FIG. 6superimposed over tracks of a servo layer of the disk media of FIG. 6 inaccordance with the present embodiment.

FIG. S, comprising FIGS. SA and SB, illustrates block diagrams ofsynchronization signal generation and processing for detection ofdowntrack vibration, spindle speed variation and jitter in accordancewith the present embodiment, wherein FIG. SA depicts a block diagram ofthe synchronization signal generation and FIG. SB depicts a blockdiagram of the synchronization signal processing.

FIG. 9 illustrates a first set of simulation results of thesynchronization signal processing of FIG. SB in accordance with thepresent embodiment.

FIG. 10 illustrates a second set of simulation results of thesynchronization signal processing of FIG. SB in accordance with thepresent embodiment.

FIG. 11 illustrates spinstand simulation results of the synchronizationsignal processing of FIG. SB in accordance with the present embodiment.

FIG. 12 illustrates a graph of a normal synchronization signal and adetected synchronization signal in a speed increase situation inaccordance with the present invention.

FIG. 13 illustrates a block diagram of position error signal (PES)generation and flying height signal generation in accordance with thepresent invention.

FIG. 14 illustrates Wallace equation generation in accordance with thepresent embodiment.

FIG. 15, comprising FIGS. 15A and 15B, illustrates a graph of touchdowncurves in accordance with the present embodiment, wherein FIG. 15Adepicts a touchdown curve using flying height signals averaged over onehundred measurements and FIG. 15B depicts a touchdown curve using aninstantaneous flying height signal measurement.

FIG. 16 illustrates a graph of experimental results for off-trackvariations of the flying height signal measured in accordance with thepresent embodiment.

FIG. 17, comprising FIGS. 17A and 17B, illustrates simulation results offlying height signals under various conditions in accordance with thepresent embodiment wherein FIG. 17A depicts the flying height signalvibration prior and acoustic emissions (AEs) and FIG. 17B depicts rootmean square values of the flying height signal as well as AE sensorsignals.

FIG. 1S illustrates a graph of a change in the flying height signal fromthermal flying height control (TFC) actuation at different altitudes inaccordance with the present embodiment.

And FIG. 19 illustrates a conventional graph of the curve of flyingheight versus TFC actuation as it varies at different altitudes.

Skilled artisans will appreciate that elements in the figures areillustrated for simplicity and clarity and have not necessarily beendepicted to scale. For example, the dimensions of some of the elementsin the block diagrams may be exaggerated in respect to other elements tohelp to improve understanding of the present embodiments. In thedrawings, reference numbers may be reused to identify similar and/oridentical elements.

DESCRIPTION

The following detailed description is merely exemplary in nature and isnot intended to limit the invention or the application and uses of theinvention. Furthermore, there is no intention to be bound by any theorypresented in the preceding background of the invention or the followingdetailed description of the invention. In this detailed description,three embodiments will be discussed: a novel and improved technique forrelative head-disk motion and spindle speed variation measurement, anovel and improved technique for downtrack synchronization in adedicated servo based magnetic recording system, and a novel andimproved technique for flying height monitoring in dedicated servo basedmagnetic recording system.

A new embodiment is presented to measure, in addition to spindle speedvariation, the relative head-disk downtrack motion. Unlike conventionalapproaches such as approaches using ball sensors and rotary encoders orapproaches using back electromagnetic field (EMF) measurement fordetermining spindle speed, the present embodiment uses written-ininformation on a disk to provide continuous and real-time informationabout relative head-disk motion, including the effects of relativehead-disk vibration. In addition, the present embodiment does notrequire additional components in a hard disk drive (HDD) implementationexcept firmware implementation of the present embodiment and a frequencysource to be provided by the HDD system on chip (SOC) circuitry.

Current measurement techniques for spindle speed are typicallyimplemented via rotary encoders (optical), hall sensors or measurementof back EMF. For a hard disk drive (HDD) which uses brushless DC (BLDC)motors and performs spindle speed measurement, such measurement can beaccomplished through Hall sensors built into the motor. In addition itis also possible, depending on the driving circuitry, to measure spindlespeed from back EMF.

However, all these methods provide only information on the rotationalspeed of the spindle and, therefore, are unable to provide the actualrelative head-disk speed or motion which is affected by other effectssuch as head-disk vibrations. In addition, the need to further increaserecording density means that current and future HDDs will move towardsvery few grains/bit recording. This puts added stress on the ability towrite accurately on the magnetic bits. The need for accurate andsynchronized writing means that spindle speed variation, jitter andundesired relative head-disk motion and vibration need to be kept verysmall. At the same time, the ability to accurately measure and quantifysuch spindle speed variation, jitter and undesired relative head-diskmotion and vibration at high resolution becomes very important.

The present embodiment presents a method and approach which can, inaddition to measuring the spindle speed and speed variations, alsoprovide measurement for downtrack relative motion and vibration betweenhead and disk at high resolution. Referring to FIG. 1, a diagram 100depicts a first approach for real time spindle speed variation andrelative head-disk motion measurement. First, a pattern 102 is writtenon a track on the disk preferably using a wide writer head. This widetrack pattern 102 will reduce the off-track interference which candegrade a readback signal 104.

The readback signal 104 is read back from the wide track pattern 102 ata frequency f106. Similar to a rotary encoder scheme, the higher thefrequency f106 used, the better the resolution. Since the achievable“encoded” resolution is limited only by a bandwidth of the read-write(RW) head and a disk media combination, a very high frequency in thehundreds of megahertz can be used, enabling an approach which is farsuperior to any conventional rotary encoder scheme. However, in order tostrike a balance between signal to noise ratio and resolution, afrequency of around 80 MHz to 150 MHz for current HDDs is preferred.

Subsequently, the RW head is positioned over the written track 102 andreads back the written track in a continuous fashion. By using areference clock signal 108 at frequency f+Δf, and mixing at a mixer 110the reference clock signal 108 with the readback signal at frequencyf112, a mixed signal having signal components at a summed frequency2f+Δf and a difference frequency Δf are obtained.

By using a filter 114 (e.g., a band pass filter (BPF) or narrow bandfilter) centered at around frequency Δf, a resulting measurement signal116 corresponding to relative spindle speed change and head-disk motionis obtained. It is possible to also use a BPF 114 that is slightlyoffset from Δf depending on the range of relative speed variation to bemeasured. The width of the filter 114 passband may also be a necessaryparameter of this embodiment. A sharper (smaller passband) filter 114will provide a larger signal change (i.e., more sensitive) for the samerelative speed change, but may suffer from a limited range of speedsthat can be measured. Thus, a good trade-off between the target range ofmeasurement versus sensitivity is desired to achieve optimal results.

The above approach depicted in the diagram 100 provides very highresolution. However, one drawback of this approach is that themeasurement signal generated is not monotonously increasing ordecreasing. Since the signal peaks at a center frequency, themeasurement may sometimes be ambiguous. This may be especially true whenthe measurement range spans to the left and right of the peak, as shownin the signal amplitude vs. RPM graphs 400, 450 in FIGS. 4A and 4B.

A second approach is depicted in diagram 200 of FIG. 2. This secondapproach alleviates any ambiguity problem that may be present in thefirst approach by using two matched filters 204, 206 instead of a singlefilter 114. The steps to generate the readback signal 112 are the sameas the first approach where a wide track of a single frequency iswritten and mixed at a mixer 110 with a reference clock signal 108 ofthe same writing frequency. However, this second approach provides theresulting mixed signal IF 202 to the two filters 204, 206, wherein thefirst filter 204 is centered at Δf+f₁, and the second filter 206 iscentered at Δf−f₁, generating the filtered signals A and B,respectively. The frequency f₁ is a frequency passband offset within thebandwidth of the first and second bandpass filters 204, 206. Anormalization procedure such as (A−B)/(A+B) can then be applied by thecombiner 208 to provide a monotonously increasing signal 210 over therange of relative spindle speeds to be measured.

Referring to FIG. 3, a graph 300 further illustrates this secondapproach. When measured signal ΔF is less than Δf302, the measuredsignal ΔF moves further into the passband of the B signal 304 and awayfrom the passband of the A signal 306. If ΔF is, instead, greater thanΔf302, the measured signal ΔF moves into the passband of the A signal306 and away from the passband of the B signal 304. Since the signal A306 must increase when the signal B 304 decreases and vice versa, theresulting signal (A−B)/(A+B) is monotonously increasing for themeasurement range between Δf+f₁ and Δf4 ₁, thereby removing theambiguity. Depending on the measurement range required, appropriate f₁values and filter widths can be chosen for optimal results.

FIG. 4A shows experimental results in the graph 400 obtained using thefirst approach for measurements on the air bearing spindle with ameasured spindle speed variation of 0.005%. FIG. 4B shows experimentalresults in the graph 450 obtained using the first approach formeasurements on a HDD product. And FIG. 5 shows experimental results inthe graph 500 when the second approach is used on a HDD product.

The technology and approaches in accordance with the present embodimentcan be easily applied to present HDD products. Because no additionalcomponents are required except a reference clock and digital filtering,both of which may be easily provided by the HDD system on chip (SOC),large additional costs of implementation are avoided. Also, byimplementing the technology into a HDD system and its associatedfirmware, it is possible to use the technology not only during HDDmanufacturing/assembly and in failure analysis situations, but alsoduring normal use by end users.

In the manufacturing and assembly stage, HDD products after assemblycould go through a long process of checks including formatting andtesting of the media. For very large drives, this could take a longtime. Due to manufacturing tolerances and production yields, not allassembled products can work at the same recording density. If the endtested product is found to be unsuitable for a certain recordingdensity, it needs to be downgraded to a lower density level. A testapproach in accordance with the present embodiment could be applied, forexample, after drive assembly and prior to the formatting and testingstep to ascertain the quality of the assembled drive. If at this stage,a drive is found to be of a “lower quality” from measured large speedvariations, jitter or relative head-disk vibrations, then the drive canbe put to a lower density level immediately. Thus, a simple measurementin accordance with the present embodiment is enabled which could helpreduce the manufacturing cost by detecting the quality of the endproduct earlier without requiring expensive and extensive testing,thereby exemplifying an intelligent manufacturing approach.

The testing approach in accordance with the present embodiment is alsouseful for the HDD under normal use by end users. For example, it ispossible to program the drive for regular self-testing using the testingapproach in accordance with the present embodiment. Such self-testingcan be performed by the HDD when the drive is idle, thereby serving as aself-reliability check during the HDD's operable life. Any variations intest results can indicate potential drive problems and future failurethat need to be fed bank to the user for remedial actions before acatastrophic failure actually occurs.

Finally, in a Failure Analysis (FA) situation, the FA process mayinclude running a test in accordance with the present embodiment andexamining the test results. A degrade in performance (e. g., increasedhead-disk vibration or variations in relative spindle speed) couldindicate the possible source of a problem to the drive engineer evenbefore a complete tear-down is warranted. Thus, the present embodimentcan also serve as a quick initial check for the FA process.

In one aspect of the present invention, a method for monitoring harddisk drive operation in a hard disk drive system including a spindle, adisk and head is provided. The method includes the steps of writing apattern having a predetermined frequency on a wide track of a hard diskdrive medium, generating a readback signal by reading the pattern fromthe track, processing the readback signal by mixing the readback signalwith a reference signal to obtain a summed signal and a differencesignal, and filtering the summed signal by a filter centered around thedifference signal to generate a measurement signal corresponding to arelative speed change of the spindle and a head-to-disk motion.

The monitoring techniques in accordance with the present embodimentadvantageously use written-in information on the disk instead of relyingon external sensors such as Hall sensors or rotary encoders, therebyenabling the actual head-disk relative speed and motion to be measuredinstead of only the spindle speed variation (conventional approachesonly measure spindle speed but cannot know relative head-diskvibration). Also, the present embodiment can be implemented in firmwareon the HDD and does not require additional components like Hall sensors.Only a frequency source is required, and such frequency source can beprovided by the HDD system on chip (SOC). Additionally, the written-in(“encoded”) track on the disk media can be at a very high frequency,providing robust, high resolution of measurements, the resolution onlybeing limited by the Read/Write (R/W) capability of the head and mediacombination. Further, the present embodiment can be used in conjunctionwith existing spindle speed control schemes in HDD, and can be appliedto current HDDs as a means for qualification of HDD components (e. g.,the spindle).

As HDD technology moves towards fewer grains/bit recording as well asfuture configurations such as Two Dimensional Magnetic Recording (TDMR)and bit-patterned media, the need to be able to determine the locationof the write and read head versus the location of individual bitsbecomes very important. Thus, the availability of a synchronizationsignal to determine these locations is paramount. When utilizing a dualfrequency dedicated servo media, where the presence of a dedicated servomagnetic layer allows “always on” servo information, the possibilityarises to obtain a “continuously on” write synchronization signal. This“continuously on” signal does not only help with accurate writes onlocation sensitive media but also provides a means to detect and measuredown track and spindle vibration, speed and other conditions.

In accordance with a second aspect of the present embodiment, a newconfiguration is proposed to make use of the frequency based dedicatedservo signal to produce an always available write synchronization signalthat can allow bit location determination, as well as detection ofdowntrack vibration, spindle speed variation, and jitter. A dedicatedservo layer 602 located below the data magnetic layer 604 is shown inthe cutaway view 600 in FIG. 6. Multi-frequency or dual frequency basedservo schemes can be utilized in the dedicated servo layer 602. A sliderhead 610 includes a reader head 612 and a writer head 614 for readingand writing to the magnetic layers 602 and 604. The slider head 610 alsoincludes a heater 616 for thermal fly height control (TFC) as known tothose skilled in the art. It is important to know the vibration, speedvariation and jitter information in order to enable synchronizedwriting.

Referring to FIG. 7, a top planar view 700 shows that the reader head612 is positioned at the middle of two tracks 702, 704 to read back thesuperposition of the signals from the two servo tracks 702, 704 in thededicated servo layer 602. The signals from the two servo tracks 702,704 are two separate frequencies as shown in a graph 706 (i.e., afrequency F1 708 and a frequency F2 710).

The process to generate the synchronization signal is given in FIG. 8,including FIGS. 8A and 8B. Referring to FIG. 8A, a block diagram 800shows that by applying analog filters or digital filters 802, 804centered at F1 and F2 frequencies 708, 710, respectively, to the readback signal 806 respective frequency components can be extracted, namelyfrequencies A and B corresponding to the signals from servo tracks f1702 and f2 704, respectively. The signals can be further processed togenerate a signal SOS that can be used for write synchronization writesynchronization in, for example, Two Dimension Magnetic Recording(TDMR), as well as for the detection of downtrack vibration, spindlespeed variation and j it ter.

The readback signal 806 from the dedicated servo system consists of botha data signal and a servo signal. Referring to FIG. SB, the F1+F2 signal820 from the filters 802, 804 (FIG. 8A) is processed using filters 822,824 (either analog bandpass or low pass filters or digital filters) toobtain the F1 and F2 components. The separate F1 and F2 signals are thenmixed at a mixer 826 and filtered by a low pass or band pass filter 828to generate a Sine (F1−F2) signal 830 which constitutes thesynchronization signal. As the synchronization signal 830 is obtainedentirely from the written-in dedicated servo signals (F1 and F2), noexternal reference clock or oscillator is required. This means that thesynchronization signal 830 avoids any phase and frequency drift that iscommon when an external reference clock is used. At the same time, thesynchronization signal 830 will fully reflect downtrack changes in theHDD system such as spindle speed variation and jitter, and relativehead-disk vibration in the down track direction.

FIG. 9 depicts three traces 902, 904, 906 that show the simulationresults for the generation of the synchronization signal S30 from thededicated servo signal. Referring to FIG. 10, simulation results aredepicted in a first trace 1000 that illustrates the synchronizationsignal 830 is resilient to relative changes in the amplitude of theservo F1 and F2 components which can occur if there is off-track in theread-write head. Essentially by detecting the zero crossing of the ACsynchronization signal, effects of off-track which can manifest forexample in a different amplitude of the synchronization signal areignored. A second trace 1050 shows that the synchronization signal 830is responsive to a relative phase difference between F1 and F2components. Referring to FIG. 11, the results 1100 further show theactual spinstand experimental results for generating the synchronizationsignal S30, Sine (F1-F2).

The synchronization signal 830 provides a means to know the location ofthe read head 612 in the downtrack direction. For example, thesynchronization signal 830 can be measured and can be translated into alocation in the downtrack direction. While a simplified approach countsthe number of zero-crossings from the start of a sector or index mark,other methods that can detect phase may also be used. The zero-crossingdetector approach is useful because it is not affected by changes in theamplitude of the synchronization signal 830. By knowing the currentlocation of the read head 612, it is possible to estimate the locationof the write head 614. With a known location of the write head 614,synchronized writing utilizing measurements in accordance with thepresent embodiment can accurately write on targeted magnetic bits.

In addition to synchronized writing, another advantageous use of thesynchronization signal 830 is that it provides information on relativehead-disk motion and captures information including undesired jitter,vibration and shock. For example, a deviation or change to thesynchronization signal 830 at a particular instant of time indicatesthat there is an event that has caused a disturbance in the downtrackdirection. Referring to FIG. 12, a graph 1200 shows an example whererelative head-disk speed suddenly increased for a short duration,resulting in a detected synchronization signal 830 that has deviatedfrom the expected or normal signal. By monitoring the synchronizationsignal 830 to detect deviations, ΔSine(F1−F2), it is possible to detectand measure events such as downtrack vibrations or changes in spindlespeed.

The ability to monitor downtrack changes and predict downtrack locationis advantageous. In the dedicated servo implementation in accordancewith the present embodiment, this is even more useful because suchmonitoring and measurement is available everywhere by virtue of the factthat servo track information is available everywhere on the disk. Byimplementing the invention into a HDD, for example through firmware inthe HDD system on chip (SOC), the monitoring and measurement can becomean important tool to detect HDD operational conditions such as excessiveshock or vibration and allow the HDD to respond quickly and takepreventive action to avoid failure and crash. Regular measurement of thevibration in accordance with the present embodiment can also help tomonitor reliability of the drive while in operation. Finally, thepresent embodiment provides a useful Failure Analysis tool to determinewhether the performance of the drive has degraded.

In accordance with this second aspect of the present embodiment, amethod for monitoring operation of a hard disk drive system having adisk medium including a buried servo layer having a first frequency ofdata on a first track and a second frequency of data on a second track,the second track adjacent to the first track, is provided. The methodincludes the steps of reading a readback signal by a head positioned atthe middle of the first track and the second track, filtering the signalby a first filter centered at the first frequency to extract a firstcomponent signal, filtering the signal by a second filter centered atthe second frequency to extract a second component signal, combining thefirst component signal and the second component signal to generate asynchronization signal comprising downtrack information. The downtrackinformation may include spindle speed variation, spindle speed jitter,and relative head-disk vibration in the downtrack direction.

In accordance with the present embodiment, written- in servo informationfor phase recovery is used. Thus, no additional information orprogramming is required. In addition, operation in accordance with thepresent embodiment does not require an external oscillator, therebyavoiding phase drift between a reference frequency and the written-inservo frequency. Also, operation in accordance with the presentembodiment is less affected by jitter and spindle speed variation sincethe (F1-F2) signal follows spindle jitter and speed variations due tousing the written-in servo information for timing. Further, thesynchronization signal S30 is resilient to crosstrack and off-trackeffects (i.e., the AC zero crossing is unaffected by the relativestrength of servo components in the dual frequency servo layer). Thechanges to the synchronization signal 830 reflect downtrack spindlejitter or speed variations, as well as relative head-disk vibration andshock. Thus, operation in accordance with the present embodiment canhelp HDD technology to achieve higher linear bits per inch recordingwhich is required to meet continued areal density growth.

In accordance with another aspect of the present embodiment, a newconfiguration is proposed to make use of a frequency based dedicatedservo signal to produce an always available Flying Height (FH) signalwith minimum off-track FH variation. This FH signal can be used as anadditional monitoring signal for contact detection and HDD reliability.

With the application of thermal Flying Height control (TFC) technology,the Wallace equation based in-situ FH testing technology becomes themajor way to measure the FH of Read/Write (R/W) heads. In theory, onesingle harmonic is good enough to detect the FH variation. But in actualapplication, the off-track of a read head changes the amplitude of areadback signal and may be misinterpreted as a change in Flying Height.This is one of the major sources of FH testing error. In order tominimize such FH testing error, a harmonic ratio method is typicallypreferred. It requires a write-in data pattern that can produce at leasttwo harmonics with harmonic signals of sufficient strength for accuratemeasurement. However, due to the special write-in pattern required onthe media, the FH value is not always available in HDDs. Further,conventional FH measurement techniques currently incorporate an acousticemission (AE) sensor or contact sensor into the magnetic read/write headof a HDD system to detect the contact point. The additional sensor(s)disadvantageously incur additional component cost for the HDD.

In accordance with a third aspect of the present embodiment, thededicated servo layer 602 in the HDD disk medium is utilized to providean always available FH signal and advantageously offers minimization ofthe effects of off-track FH variation. This FH signal can be used as anadditional monitoring signal for contact detection and HDD reliability.Referring back to FIG. 6, the dedicated servo layer 602 is located belowthe date magnetic layer 604 in the disk medium. The flying height (FH)630 refers to the height of the head 610 as it flies over the diskmedium. Multiple frequencies or a single frequency of data are stored onthe servo tracks of the servo layer 602. The reader 612 is positioned atthe middle of two tracks 702, 704 to read back the superposition of thesignal as shown in FIG. 7. Referring to the block diagram 1300 of FIG.13, by applying analog or digital filters 802, 804, the signal for therespective frequency components can be extracted from the readbacksignal 806, and the amplitude of the signals, namely A and B can bedetermined. The amplitude of the signals can be further processed toproduce a position error signal (PES) 1302, the synchronization signal808 and a flying height signal 1304.

Based on the Wallace Spacing Loss equation, the spacing Loss isexpressed as:

A=e ^(−2πd/λ)

where λ is the wavelength of the written data pattern and d is therelative change in spacing. Based on the amplitude ratio of the Wallaceequation, we can use five different Wallace spacing methods (obtainedfrom equation (I)) to calculate FH from the simultaneous two frequencyservo signal. They are:

$\begin{matrix}{{\Delta \; d} = {- \frac{\Delta \; {\ln (A)}*\lambda_{A}}{2\pi}}} & (2) \\{{\Delta \; d} = {- \frac{\Delta \; {\ln (B)}*\lambda_{B}}{2\pi}}} & (3) \\{{\Delta \; d} = {{- \frac{\Delta \; {\ln ({AB})}}{2\pi}}{\frac{\lambda_{A}\lambda_{B}}{\lambda_{B} + \lambda_{A}}}}} & (4) \\{{\Delta \; d} = {{- \frac{\Delta \; {\ln \left( {A/B} \right)}}{2\pi}}{\frac{\lambda_{A}\lambda_{B}}{\lambda_{B} - \lambda_{A}}}}} & (5) \\{{\Delta \; {\ln \left( {A/B} \right)}} = {\ln {{^{\frac{2\; {nd}}{\lambda_{A}}} + ^{\frac{2\; {nd}}{\lambda_{B}}}}}}} & (6)\end{matrix}$

The above equations are based on ln(A), ln(B), ln(A*B), ln(A/B) andln(A+B). The respective equations are shown in FIG. 14. It isstraightforward to calculate FH based on the first four equations.However, for the case ln(A+B), a more complex equation is needed tocalculate FH and d needs to be calculated from a trial and error oriterative approach.

Alternatively, a simple calibration approach can be taken to determinethe FH instead of solving the complex equation (6). In this case, we usethe ln(A*B) case to calibrate FH for In(A+B). The calibration equationis shown below:

$\begin{matrix}{{\Delta \; d} = {\frac{\Delta \; {\ln \left( {A + B} \right)}}{\Delta \; {\ln \left( {A + B} \right)}_{x}}x\; \Delta \; d_{x}}} & (7)\end{matrix}$

where x is the relative TFC applied and Δd_(x) is the correspondingrelative FH measured by Δln(A*B)_(x). By normalizing the Δln(A+B)against Δln(A+B)_(x), the FH can be deduced.

FIG. 15, including FIGS. 15A and 15B, shows the touch down curve forrespective methods of FH measurement. A graph 1500 in FIG. 15A shows theFH measurement after one hundred times averaging, while a graph 1550 inFIG. 15B indicates an arbitrary instantaneous one point FH measurement.The information depicted in the graph 1550 shows that the FH measured bya single frequency is greatly affected by off track error. This isfurther confirmed by the experimental results shown in a graph 1600 ofFIG. 16. For these results, the TFC was fixed. It can be seen that theoff-track FH variation error can be systematically studied by moving thereader sensor from −20 nm off-center to +20 nm off-center. However,ln(A−B) shows very little off-track FH variation error.

The instantaneous and always available FH signal can also be used as thecontact detection sensor. FIG. 17A shows the time domain FH signal. InFIG. 17A, a graph 1700 shows FH signals 1705 and 1715 at different TFCpowers. The FH signal 1715 shows a FH signal during normal operationwhile the vibration FH signal 1705 is clearly observed during the headdisk contact at a higher TFC power. FIG. 17B depicts a graph 1720 whichplots the root mean square (RMS) of the FH signal at different TFCpowers where signals 1722, 1724, 1726 are obtained using ln(A+B), ln(B)and ln(A), respectively. In the graph 1720, the RMS measurements depicta clear sudden increase of value at contact TFC. In some cases, the FH'sRMS can even detect the FH vibration prior the AE sensor as shown in agraph 1800 of FIG. 1S. This early detection can be attributed topre-contact slider vibration. All of the above-mentioned FH measurementmethods can be used as contact detection. Together with the continuousPES signal 1302, these give a two dimensional and fast response to nearcontact condition.

As the FH versus TFC actuation curve is different at different altitudes(as shown in a prior art example in graph 1900 of FIG. 19), it ispossible to create different FH calibration curves for differentaltitudes. One way is to calibrate and obtain curves under different airpressures corresponding to the different altitudes.

Subsequently, in actual HDD operation, by moving the operating pointthrough changing the TFC actuation (without going to contact) andmeasuring the change in FH for each TFC power used, it is possible todetermine the gradient, ΔFH/ΔPower_(TFC). This parameter can be used todetermine on which altitude curve the drive is operating. If thealtitude curve is correctly determined, then appropriate FH look-uptables could be used and unnecessary and undesired head disk contactcould be avoided by being able to apply the appropriate TFC power fordifferent altitudes.

The ability to determine the altitude curve on which the drive isworking is of great value. In present HDDs, it is difficult toincorporate an altitude sensor. Although drives have temperature sensors(e.g., thermistors), they generally do not have altitude sensors. Thismeans that current HDDs do not know whether they are being operated at ahigh altitude (e.g., greater than twenty thousand feet) or at sea level.When at a high altitude, the flying height of the head is lower andthere is increased chance of head-disk contact and reliability issues ifan improper TFC power (such as that suitable for sea level use) isapplied.

Appropriate look-up tables for HDD operation under different conditionsof pressure (altitude) and temperature can be generated by operation inaccordance with the present embodiment. Once these tables are generated,it is possible in accordance with the present embodiment to identify theright look up table to be used in actual drive operation, thus reducingthe reliability problems associated with HDD usage at differentaltitudes. Without calibrated look-up tables for different altitudes,the common approach for current HDDs is to spin down the disk or adjustthe TFC such that the head/slider comes into intermittent contact withthe disk. Once this is done, the flying height is then known from theamount of TFC actuation applied. However, this touch down approach maywear out the RW heads and also has the risk of head disk damage.Therefore, reduction or elimination of the need to touch down the RWhead is enabled by the present embodiment.

Another important advantage of the present embodiment is that the FHinformation is always available since the servo information is availableeverywhere for the dedicated servo HDD. Compared to current drives whereFH measurement can only depend on a few designated and specially writtenareas on the disk (if readback signal approach to determine FH is used),the always available FH information for the dedicated servo disk comesat little cost, but brings about great benefits.

In accordance with one aspect of the present invention, a method formeasuring flying height of a head over a disk medium in a hard diskdrive (HDD) system having a disk medium including a buried servo layerhaving a first frequency of data on a first track and a second frequencyof data on a second track, the second track adjacent to the first track,is provided. The method includes the steps of reading a readback signalby a head positioned at the middle of the first track and the secondtrack, filtering the signal by a first filter centered at the firstfrequency to extract a first component signal, filtering the signal by asecond filter centered at the second frequency to extract a secondcomponent signal, combining the first component signal and the secondcomponent signal using Wallace equations to generate a synchronizationsignal comprising flying height information. A second aspect includesdeveloping a table of operational parameters for various pressures andtemperatures from the flying height information for improved HDD systemoperation.

Thus it can be seen that methods for hard disk drive system operationhave been disclosed which provides many advantages over the drawbacks ofconventional HDDs. While several exemplary embodiments have beenpresented in the foregoing detailed description of the invention, itshould be appreciated that a vast number of variations exist, includingvariations as to the materials, structure and operation of the datastorage device.

It should further be appreciated that the exemplary embodiments are onlyexamples, and are not intended to limit the scope, applicability,dimensions, or configuration of the invention in any way. Rather, theforegoing detailed description will provide those skilled in the artwith a convenient road map for implementing an exemplary embodiment ofthe invention, it being understood that various changes may be made inthe function and arrangement of elements and method of play stepsdescribed in an exemplary embodiment without departing from the scope ofthe invention as set forth herein.

What is claimed is:
 1. A system comprising: a write head configured towrite a pattern on a track of a medium of a hard disk drive, wherein thepattern has a first frequency; a read head configured to read thepattern written on the track of the medium of the hard disk drive; amixer configured to mix a first signal generated by reading the patternwith a second signal to obtain a mixed signal, wherein the second signalhas a second frequency, and wherein the second frequency is differentthan the first frequency; and a filter configured to filter the mixedsignal to determine a relative head-to-disk speed, wherein the filterhas a parameter selected based on a difference between the firstfrequency and the second frequency.
 2. The system of claim 1, whereinthe write head of the hard disk drive is wider than the read head of thehard disk drive.
 3. The system of claim 1, wherein the mixed signalincludes signal components having frequencies at a sum of the firstfrequency and the second frequency and at a difference of the firstfrequency and the second frequency.
 4. The system of claim 1, whereinthe parameter of the filter includes a center frequency of the filterselected based on the difference between the first frequency and thesecond frequency.
 5. The system of claim 1, wherein: the parameter ofthe filter includes a center frequency of the filter that is offset by apredetermined frequency from the difference between the first frequencyand the second frequency; and the predetermined frequency is selectedbased on a range of variation of the relative head-to-disk speed to bemeasured.
 6. The system of claim 1, wherein the parameter of the filterincludes a width of a passband of the filter selected based on one ormore of sensitivity and range of variation of the relative head-to-diskspeed to be measured.
 7. The system of claim 1, wherein the firstfrequency between 80 MHz and 150 MHz.
 8. The system of claim 1, wherein:the filter includes a first filter having a first center frequencyselected based on the difference between the first frequency and thesecond frequency; the filter includes a second filter having a secondcenter frequency selected based on the difference between the firstfrequency and the second frequency, wherein the second center frequencyis different than the first center frequency; and the relativehead-to-disk speed is determined based on outputs of the first filterand the second filter.
 9. The system of claim 8, further comprising acombiner configured to determine the relative head-to-disk speed bynormalizing the outputs of the first filter and the second filter. 10.The system of claim 8, wherein: the first center frequency is selectedby adding a frequency offset to the difference between the firstfrequency and the second frequency; the second center frequency isselected by subtracting the frequency offset from the difference betweenthe first frequency and the second frequency; and the frequency offsetis selected within a bandwidth of the first filter and the secondfilter.
 11. A method comprising: writing, using a write head of a harddisk drive, a pattern on a track of a medium of the hard disk drive,wherein the pattern has a first frequency; reading, using a read head ofthe hard disk drive, the pattern written on the track of the medium ofthe hard disk drive; mixing a first signal generated by reading thepattern with a second signal to obtain a mixed signal, wherein thesecond signal has a second frequency, and wherein the second frequencyis different than the first frequency; selecting a parameter of a filterbased on a difference between the first frequency and the secondfrequency; and filtering, using the filter, the mixed signal todetermine a relative head-to-disk speed.
 12. The method of claim 11,wherein the mixed signal includes signal components having frequenciesat a sum of the first frequency and the second frequency and at adifference of the first frequency and the second frequency.
 13. Themethod of claim 11, wherein the selecting the parameter of the filterincludes selecting a center frequency of the filter based on thedifference between the first frequency and the second frequency.
 14. Themethod of claim 11, wherein the selecting the parameter of the filterincludes selecting a center frequency of the filter that is offset by apredetermined frequency from the difference between the first frequencyand the second frequency.
 15. The method of claim 14, further comprisingselecting the predetermined frequency based on a range of variation ofthe relative head-to-disk speed to be measured.
 16. The method of claim11, wherein the selecting the parameter of the filter includes selectinga width of a passband of the filter based on one or more of sensitivityand range of variation of the relative head-to-disk speed to bemeasured.
 17. The method of claim 11, further comprising: selecting thefirst frequency is between 80 MHz and 150 MHz; and writing the patternusing the write head that is wider than the read head of the hard diskdrive.
 18. The method of claim 11, wherein the filtering the mixedsignal using the filter includes: filtering the mixed signal by a firstfilter having a first center frequency selected based on the differencebetween the first frequency and the second frequency; filtering themixed signal by a second filter having a second center frequencyselected based on the difference between the first frequency and thesecond frequency, wherein the second center frequency is different thanthe first center frequency; and determining the relative head-to-diskspeed based on outputs of the first filter and the second filter. 19.The method of claim 18, further comprising determining the relativehead-to-disk speed by normalizing the outputs of the first filter andthe second filter.
 20. The method of claim 18, further comprising:selecting a frequency offset within a bandwidth of the first filter andthe second filter; selecting the first center frequency by adding thefrequency offset to the difference between the first frequency and thesecond frequency; and selecting the second center frequency bysubtracting the frequency offset from the difference between the firstfrequency and the second frequency.